In an electrically corrosion-proofing process of this type, it is a conventional practice to set the polarity of a metal member at negative to supply electric current continuously or intermittently between the metal member and an electrode.
With this electrically corrosion-proofing process, the metal member is maintained at a high potential. For this reason, if a damaged portion reaching the metal member exists in the coating film, when electric current flows in an exposed portion of the metal member in the damaged portion, a reducing reaction occurs in the exposed portion, and hence, the corrosion of the exposed portion can be prevented.
With the conventional process, however, the following problem arises: OH ion produced by the reducing reaction reduces the adhesion force of the coating film to the metal member from a starting point provided by the damaged portion of the coating film. For this reason, the peeling-off of the coating film is produced, and the width of peeling-off of the coating film is increased substantially in proportion to the current supply time.
For example, Japanese Utility Model Application Laid-open No.2-106465 suggests a technique in which a mounting bore is provided in a metal member having a coating film which is to be brought into contact with a liquid, and an electrode is inserted into the mounting bore with a cylinder-shaped insulator interposed therebetween, whereby DC current is allowed to flow between the metal member and the electrode to provide a corrosion-proofing of a peeled-off portion produced in the coating film, and proposes that a covering member is mounted on the insulator for sealing a corner of the mounting bore in which a defect such as a pinhole is liable to be produced, from a surrounding liquid. This ensures that the corner of the mounting bore in which the electrode is mounted, can be shielded from the surrounding liquid by the covering member to prevent the peeling-off of the coating film in the corner, but this technique is not the one for effectively inhibiting the progressing of the peeling-off of the coating film produced in a portion other than the corner of the mounting bore.
Japanese Patent Application Laid-open No.61-221382 discloses the technique intended to flame-spray copper or a copper alloy onto a surface of a steel construction submerged in the sea water such as a boat or ship, and to allow DC current to flow to the steel construction and an electrode of copper or a copper alloy disposed as being opposite from the polarity of the construction, while alternately inverting the polarities of the steel construction and the electrode, thereby carrying out a corrosion-proofing treatment. More specifically, the object of this patent publication is in that in the state in which the polarity of the steel construction has been set at negative, the corrosion-proofing treatment of the steel construction is carried out, but in the state in which the polarity of the steel construction has been set at positive, copper ion is caused to be eluted from the copper-sprayed steel plate of the construction, thereby inhibiting the pollution of the copper-sprayed steel plate due to the deposition and growth of a shellfish or other sea creatures thereto by toxicity of the copper ion. This patent publication does not at all disclose a technique for effectively inhibiting the progressing of the peeling-off of the coating film of the copper or copper alloy flame-sprayed onto the steel construction, either.